Typically, in a solid-state relay, an input circuit and an output circuit are electrically isolated from each other by a photo-coupler, and a load is selectively connected and disconnected to the output circuit according to a signal supplied to the input circuit.
In a solid-state relay of this type, a semiconductor switching element such as a triac has been typically used and FET's have lately come to be preferred as such a switching element. This is because an FET presents comparatively less electric resistance when it is in a conductive state than a triac in the same state and therefore generates less heat. As a result, an FET requires smaller heat dissipating fins and can reduce the overall size of the solid-state relay.
FIG. 3 shows a conventional solid-state relay circuit which makes use of an FET.
In the drawing, a resistor 3 for limiting current and a light emitting diode 4 of a photo-coupler 6 are connected in series across a pair of input terminals 1 and 2. The photo-coupler 6 further comprises a plurality of photo-diodes 5 which are connected in series with each other and are optically coupled to the light emitting diode 4. An input circuit 100 and an output circuit 200 are electrically isolated from each other by this photo-coupler 6. Numeral 7 denotes a resistor which is connected in parallel with the photo-diodes 5.
A pair of FET's 8 and 9 are connected in series with each other, with their sources connected in common. The drains of the FET's 8 and 9 are connected to a load 12 and an AC power source 13 in such a manner that the load, the power source 13 and the FET's 8 and 9 are connected in series in a closed circuit. The cathode of a diode 10 is connected to the drain of the FET 8 and the anode of the diode 10 is connected to the source of the FET 8. Likewise, the cathode of a diode 11 is connected to the drain of the FET 9 and the anode of the diode 11 is connected to the source of the FET 9.
The sources of the FET's 8 and 9 which are connected in common are further connected to the anode of the photo-diodes 5 and the gates of the FET's 8 and 9 are connected in common to the cathode of the photo-diodes 5.
The action of the circuit of FIG. 3 is as follows:
When there is no input signal applied across the input terminals 1 and 2, the light emitting diode 4 does not light up and therefore no electromotive force is produced in the photo-diodes 5. Therefore, the FET's 8 and 9 are both off and no electric power is transmitted to the load 12.
When an input signal is applied across the input terminals 1 and 2, electric current is supplied to the light emitting diode 4 and the light emitting diode 4 lights up. This light is transmitted to the photo-diodes 5 by way of the photo-coupler 6, producing an electromotive force in the photo-diodes 5. As a result, a certain electric voltage is produced across the resistor 7 and applied across the sources and the gates of the FET's 8 and 9, respectively.
Therefore, the FET 8 turns ON during the positive half period of the power source 13 and electric power is transmitted to the load 12 by way of the FET 8 and the diode 11 which is connected across the source and the drain of the FET 9. During the negative half period of the power source 13, the FET 9 turns ON and electric power is transmitted to the load 12 by way of the FET 9 and the diode 10 which is connected across the source and the drain of the FET 10. Thus, the load receives electric power irrespective of the polarity of the power source at each moment and remains in this state throughout the time a signal is supplied to the input circuit 100.
FIG. 4 is a waveform diagram showing the above described action; FIG. 4 (a) shows a power source voltage, FIG. 4 (b) shows an input signal applied across the input terminals 1 and 2, and FIG. 4 (c) shows the electric current flowing through the load 12.
However, according to this conventional circuit, since the FET's 8 and 9 turn off immediately after the input signal has disappeared and the electromotive force of the photo-diodes 5 has been lost, if the input signal turns OFF near a peak P of the power source voltage waveform shown in FIGS. 4 (a) and 4 (c), the load current abruptly turns OFF. Therefore, if the load 12 includes an inductive component, a counterelectromotive force is produced and this could cause destruction of the FET's 8 and 9 and the diodes 10 and 11 and generation of electric noises which may interfere with radio communication.